I always use SamSpade.org for quick lookups. When I get curious of who's server its on and who else is on it I also use one of my websites LookupDomainWhois.com which has a script on it I got from here that shows other domains on an IP.
I use whois.sc . I have never had the normal server errors that come from so many other sites. It also gives you alot of information about the site.
Generally i use the command "whois" on my system, but sometimes when i want some extra information i use whois.sc